to HOME page
Soil
 
                   
 
 
Historical renditions of the mighty earth movers
Some background snippets
What are some of the benefits of encouraging earthworm

Different earthworms have different
homes and different jobs

What affects earthworms?
Do I have enough earthworms in my garden?

How do I encourage earthworms in my garden?

 

Earthworms

Earthworms are segmented worms that are scientifically classified as belonging to the phylum Annelida (ringed animal). There are 3500-4000 species of earthworms around the world. They are full of calcium, protein, fibre and vitamins, making them a valuable food source for many mammals, reptiles and fish. Earthworms vary in size, on average from no more than 1 centimetre to about 3 metres in length. One of the world’s largest earthworms, the Giant Gippsland Earthworm (Megascolides australis), is found in Australia. It has an average length exceeding 1 metre. However, the longest recorded earthworm was a South African giant specimen (Microchaetus rappi), measuring around 7 metres in length.

Historical renditions of the mighty earth movers

As they go about their daily business, largely obscured and forgotten by the world above them, earthworms have been shown to consume and have important effects on vast amounts of the soil beneath our feet. It was just a few months after the British scientist Charles Darwin returned from his famous voyage to the Galapagos Islands in 1837 that he went to visit his uncle in the country. As they walked in the garden, the uncle pointed out a spot where he had spread ashes and lime several years earlier. The spot had become buried in soil cast by earthworms. Astonished to see that worms could move so much soil, Darwin went home, lined the shelves of his study with glass-covered pots full of earth and worms and began a series of experiments that lasted 40 years. In one of the long experiments the scientist demonstrated his patience. Darwin spread a layer of chalk on a grassy field and waited 29 years to come back and dig a trench to see how deep the earthworm castings piled up. He found the chalk was now sitting below 6 inches (about 15 cm) of soil! In 1881 Darwin remarked “it may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures”.

Meanwhile in New Zealand in 1925 a farmer, Mr A S Ashmore, from Raetihi became convinced that earthworms were not only moving large amounts of soil around, but that their activities were having a beneficial effect on the growth of the pasture on his farm. He was so convinced by his observations that he began to deliberately introduce earthworms to areas of his farm previously devoid of earthworms. Later scientists were able to confirm Mr Ashmore’s observations when they calculated that the introduction of earthworms had in the long term improved the overall productivity of the pasture by 25-30%. Encouraging earthworm activities in the soil meant that the farm was now able to carry 2.5 stock units per hectare more than it could before – a clearly very beneficial effect. This research sparked off a series of pioneering earthworm experiments by a number of New Zealand scientists that ultimately have vastly improved the current global understanding of earthworm ecology.

to top of page

Some background snippets

There are 3500-4000 species of earthworms in the world and nearly 200 species have been identified in New Zealand.

  • They are found in all but the driest and coldest parts of the world, but generally prefer moist (but not too wet) environments.
  • Pastures commonly support the biggest populations of earthworms as they usually contain large amounts of organic matter and are infrequently disturbed by cultivation events.
  • Numbers of earthworms can commonly range between 7 and 12 million per hectare under a productive pasture in New Zealand. This corresponds to 1 to 3 tonnes of earthworms per hectare and means that the weight of earthworms below a pasture is similar to the weight of the grazing animals supported above ground!
  • Earthworms are hermaphroditic, which means they each have both male and female organs.
    It is an old wives’ tale that cutting an earthworm in half will make two earthworms…. one part may survive, but it is much more likely that both parts will die.
  • The excreta produced by earthworms is known as casts. As soil passes through an earthworm’s body, some of the nutrients are converted into forms that are more readily available for plant uptake. So casts are generally rich in plant-available nutrients like nitrogen, phosphorus, and potassium.
  • As well as consuming soil, earthworms also help to break down and thereby recycle organic materials (such as dead herbage and dung). Scientists have calculated that each year through their activities they are responsible for burying around 6 tonnes of pasture litter per hectare.
  • As they burrow through the soil, earthworms create burrows and channels that help to loosen the soil, allowing air to circulate and roots and water to penetrate the soil more easily.
  • In New Zealand an incredible 25-30 tonnes of soil per hectare per year has been measured by scientists as being deposited by earthworms on the soil surface in the form of casts. But we must remember that some earthworms deposit their cast material both within and upon the soil surface, so the total amount of soil that they turn over in a year is even higher!

to top of page

 

What are some of the benefits of encouraging earthworms activity in soil?

 

  • improved physical structure of the soil
  • better drainage and aeration
  • enhanced soil fertility (nutrients become more readily available to plants after they have been consumed by earthworms)
  • surface litter incorporation and recycling of nutrients back into the soil
  • better water infiltration / reduced run-off of water
  • improved root penetration

 

Different earthworms have different
homes and different jobs

 

It is important to realise that not all earthworms are the same – individual species have individual roles to play and functions to carry out.

There are four main types of earthworms that you might find examples of in your garden:

  • Compost dwellers. Like to live in high organic matter environments such as compost heaps, but will not usually survive in soil unless it contains very high amounts of organic matter.
  • Soil surface dwellers. Feed on decaying roots, shoots, leaves and dung and live near the soil surface (0-15 cm depth). Important in mixing plant litter into the soil.
  • Topsoil dwellers. Most common earthworms in New Zealand; live in the top 20-30 cm depth of soil. Burrow through soil, eating and excreting it. Tend to eat more soil than organic matter.
  • Subsoil dwellers. Tend to live in permanent burrows up to as deep as 3 metres below soil surface. Drag food such as leaves into their burrows from the soil surface. Often larger than other earthworm types.

 

to top of page

 

 

Examples of some earthworm species,
from www.crop.cri.nz

 

 

Compost preferring or soil surface dwelling species
These species tend not to burrow into the soil. They prefer to live at or near the soil surface or in compost heaps since they like to eat material that is high in organic matter, e.g. decaying plant roots and shoots, dung and leaves. Examples of such species include the dung worm Lumbricus rubellus and the tiger worm Eisenia fetida.
E. Fetida L. rubellus
 
Topsoil dwelling species
These species usually live in the top 20-30 cm depth of soil. They burrow through the soil, ingesting it as they go and thus mix the topsoil layer (see diagram). As they burrow they produce stable earthworm casts that help to improve soil structure and increase soil aeration by creating cracks and channels in the soil. The most common earthworm found in New Zealand is a topsoil dwelling species called the grey worm, Aporrectodea caliginosa. It is the most dominant species in our gardens.
A. caliginosa O. cyaneum
 
Earthworm hole
The burrowing activity of earthworms creates burrows and channels (worm holes) throughout the soil. These help to loosen the soil, improving soil structure and fertility, and create spaces for roots to grow through and air, water and fertiliser to enter.
Earthworm hole

 

to top of page

 

 

 

What affects earthworms?

 

  • temperature (they don’t like it too hot or too cold)
  • moisture (they don’t like it too wet or too dry)
  • food availability/type (some sources of organic matter are of better quality/contain more nutrients than others)
  • soil type and texture (soil organic matter is a good food source; sand can be abrasive to the earthworm’s skin)
  • pH of soil/organic material (most earthworms prefer a pH closer to neutral)
  • seasonality (numbers decline during summer when it gets hot and dry)
  • land management (cultivation, fertiliser application, irrigation)
  • predators (such as birds and flatworms)
  • toxic substances (such as fungicides and fumigants)

to top of page

 

Do I have enough earthworms in my garden?

There are a number of tell-tale signs that indicate you may have a lack of earthworms:

  • a turf mat of decomposing peat-like material may form near the soil surface
  • old plant material remains on the soil surface and does not break down
  • additions of fertiliser or lime to the soil surface do not get mixed into the soil

 

How do I encourage earthworms in my garden?

 

  • Maintain soil pH between 5.8 and 6.3 by adding lime periodically to the soil. Limit the amount of cultivation where possible but, if cultivating, avoid machines that pulverise the soil and the earthworms contained therein. Limit the use of harmful pesticides (particularly fungicides and fumigants).
  • Irrigate the soil during dry periods to maintain earthworm activity, and increase organic matter in the soil (a good food source for earthworms) by incorporating composted material or animal manure.

to top of page